This invention is directed to fluids used for the drilling of wells. In particular, this invention provides a drilling fluid comprising a polar organic compound, as well as water soluble potassium or ammonium salts of organic acids. When mixed in proper proportions, these two components will provide a drilling fluid which is not toxic, contains no chloride, and is inhibitive with respect to clay hydration and swelling. The fact that it is of no or low toxicity makes it extremely useful in that it reduces environmental concerns. Its inhibitive qualities result in the reduction of water transferring to the shale defining the wellbore, yielding in an increased stability of water sensitive shales.
It is well known in the art that drilling fluids must be used in connection with the drilling of wells, such as those wells used in the oil and gas industry. Such drilling fluids serve several functions in the drilling process. These functions include: removal of drilled cuttings, sealing of the sides of the well bore so as to minimize drilling fluid loss into the formation, provision of a hydrostatic head to prevent blowouts from high pressure fluids entering into the wellbore and flowing to the surface, creation of a low-friction surface on the well bore to facilitate rotation and removal of the drill string as operational conditions require, cooling of the drill bit, and lubrication to prevent the drill pipe from sticking during rotation.
Drilling fluids have typically been colloidal suspensions of certain viscosifiers and filtration control materials, such as clays, as well as of fine drilled solids, in either oil or water, as is well known in the art. Various chemicals are added to alter, enhance, influence or modify the properties of this suspension, using techniques which are known to one skilled in the art. For example, a weighting agent, such as barium sulfate, commonly known as "barite," is added to increase the density of the fluid. Gellants are used to increase viscosity and gel strength and provide suspension properties. Filtration control materials, such as soluble polymers or asphaltic materials, are added to encourage the development of the filter cake on the sides of the wellbore so that a minimal amount of the drilling fluid will enter a permeable formation.
One of the concerns in using a water-based drilling fluid is that, often, the shales through which the well is being drilled are very sensitive to water. These shales absorb water easily and such absorption renders them highly unstable. Thus, when the shales become hydrated, they tend to create an unstable wellbore, frequently resulting in a collapse of the wellbore. This difficulty renders the use of water-based drilling fluids undesirable in such shales. Shale stabilization is believed to be at least partially dependent upon inhibition of swelling of the shale, and dispersion of the shale into the fluid. The amount of swelling to be experienced varies from shale type to shale type. By "inhibition" is meant the ability of a process or a drilling fluid to retard hydration of the shale adjacent to the wellbore whereby the shale remains intact and basically in its original size, shape and volume, said process being the manufacture of an inhibitive fluid suspension and using that suspension as a drilling fluid. In such cases, oil-based drilling fluids are frequently used in an effort to control the water in the drilling fluid, and, thus, to minimize shale destabilization. However, as the environmental impact of the disposal of these oily spent slurries, and the drilled cuttings carried by these slurries, has become increasingly scrutinized, inhibitive water-based fluids have become more and more the fluid of choice in the industry. Inhibitive water-base drilling fluids that contain potassium chloride have been used as alternatives to oil-base systems. However, environmental concerns about high levels of chloride ions have limited disposal of spent fluid in onshore drilling operations. Environmental concerns about potassium chloride toxicity at high concentrations to mysid shrimp have limited disposal of spent fluid and drill cutting in offshore drilling operations.
The drilling fluid of this invention addresses these competing concerns of environmental acceptability of the drilling fluid on the one hand and operational acceptability of the circulating drilling fluid on the other. These concerns are resolved by using a polar organic fluid in conjunction with the potassium or ammonium salt of an organic acid.
The use of polar organic fluids, such as glycerine, in conjunction with certain cations, such as potassium or ammonium salts of organic acids, results in an interaction with the clays in shales to lower their ability to hydrate and swell. This means that the fluid will not interact, or will interact only slightly, with the shales. The net result of this low interaction is that the shales will remain stable. The fact that this is performed without the use of either oil or chloride ions renders the spent drilling fluids, and the drilled cuttings resulting therefrom, safely disposable from an environmental standpoint.
Conventional drilling fluids of the related art, which fluids reduce hydration of the shale formations surrounding the wellbore, are potassium chloride in water-base polymer systems as described by Steiger in Fundamentals and Use of Potassium/Polymer Drilling Fluids to Minimize Drilling and Completion Problems Associated With Hydratable Clays published in the August 1982 issue of the Journal of Petroleum Technology. Similarly, U.S. Pat. No. 4,830,765 to Perricone et al. discloses a non-polluting inhibitive drilling fluid which contains water and a water-soluble component selected from the class consisting of polyhydric alcohols, glycol, glycol ethers, polypropylene, polyethylene glycols, ethylene oxide-propylene oxide copolymers, alcohol initiated ethylene oxide-propylene oxide copolymers and/or mixtures thereof, wherein the ratio of said water soluble component in the total liquid phase was between about 5% to about 50% by volume. However, in the Perricone patent, the unexpected benefit of the addition of potassium or ammonium salts of organic acids to the fluid was not recognized.
U.S. Pat. No. 4,780,220, to Peterson, another drilling fluid which uses glycerine is disclosed. Again, that patent does not recognize the benefit of adding potassium or ammonium salts of organic acids to the drilling fluids in addition to the glycerine.
Therefore, while different water soluble synthetic additives, including polar organic fluids, have been used in the petroleum industry in drilling fluids in the past, their use in a drilling fluid in conjunction with potassium or ammonium salts of organic acids has heretofore not been appreciated by those skilled in the art.
The present invention comprises a drilling fluid having a liquid phase which imparts lubricity and shale swelling inhibition properties to the fluid similar to the beneficial properties of oil-based drilling fluids without the accompanying adverse effects, such as high toxicity. These and other benefits of this invention will become apparent to one skilled in the art by review of this specification, examples and claims hereto.